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Full text of "Space environmental : topics : aurora / [written by Larry Combs and Rodney Viereck]"

C 55.2: AU 6 





Space Environment 

TOPICS 



Pennsylvania State University 
Libraries 

OCT 3 1 1996 

Documents Collection 
U.6. Depository Copy 



Space Environment Center 

325 Broadway, Boulder, CO 80303-3326 

(303) 497-5127 



SE-12 



Aurora 



TF 



he beautiful and often eerie curtains 
of light in the night-time sky known 
as aurora have been enjoyed by 
people for millennia. Called the au- 
rora borealis or "northern lights" (fig. 1), auro- 
ra also occur in the Southern Hemisphere and 
are called the aurora australis. 



Many legends, myths and superstitions have re- 
volved around the aurora throughout the histo- 
ry of mankind. The early dragon legends of 
China and Europe are said to have originated 
from the aurora. Some cultures have regarded 
the sighting of the aurora as a sign of royal 
birth; to others it suggests ghosts of the dead or 




Figure 7.— Aurora borealis taken in the Copper River Delta, Alaska; ®1990 by Dave Parkhurst, Alaska Naturally. 



the precursor for war. The Eskimos of North 
America believed that if you whistled at the au- 
rora it would sweep down and take you from the 
earth; by clapping your hands you could force 
it to retreat. 

Magnets Are the Key 

The origin of the aurora is 93 million miles ( 149 
million km) from Earth at the Sun. Energetic 
particles from the Sun are carried out into space 
along with the ever-present hot solar wind. This 
wind sweeps supersonically toward Earth 
through interplanetary space at speeds ranging 
from 300 to over 1000 km per second, carrying 
with it the solar magnetic field. The solar wind 
distorts the Earth's magnetic field to create the 
comet-shaped magnetosphere (fig. 2). 

The terrestrial magnetic shield acts as a barrier, 
protecting the Earth from energetic particles 
and radiation in the hot solar wind. Most of 
these energetic particles are deflected around 
the Earth by the magnetosphere, but some get 
trapped. Electrons trapped in the Earth's mag- 
netic field are accelerated along the magnetic 



field toward the polar regions and then strike 
the atmosphere to form the aurora. 







m J Auroral oval 


j^r& 




^®ogrstbhic 


1 *£■ 


"Magnetic 


o»5orth^\ 


fl .-». ^ 


North 


u^roA 



Figure 3. Energetic electrons spiral down the geo- 
magnetic field lines towards the polar regions, strik- 
ing the upper atmosphere, resulting in the display of 
auroral lights. 

The particles, which stream down the magnetic 
field of the Earth, reach the neutral atmosphere 
in a rough circle called the auroral oval. This 
circle, or annulus, is centered over the magnetic 
pole and is around 3000 km in diameter during 
quiet times. The annulus grows larger when the 
magnetosphere is disturbed. The location of the 
auroral oval is generally found between 60 and 
70 degrees north and south latitude (fig. 3). 



Solar Wind 




Acceleration 
Region 



Figure 2. — A "side view" of the Earth and magnetosphere showing some of the important regions. 



Auroral features come in many shapes and 
sizes. Tall arcs and rays start brightly 100 km 
above the Earth's surface and extend upward 
along its magnetic field for hundreds of km. 
These arcs or curtains can be as thin as 100 me- 
ters while extending from horizon to horizon. 
Auroral arcs can nearly stand still and then, as 
though a hand has been run along a tall curtain, 
the aurora will begin to dance and turn. After 
midnight, the aurora can take on a patchy ap- 
pearance and the patches often blink on and off 
once every 10 seconds or so until dawn. 

Most of the auroral features are greenish yellow 
but sometimes the tall rays will turn red at their 
tops and along their lower edge. On rare occa- 
sions, sunlight will hit the top part of the auroral 
rays creating a faint blue color. On very rare oc- 
casions (once every 10 years or so) the aurora 
can be a deep blood red color from top to bot- 
tom. In addition to producing light, the energet- 
ic auroral particles deposit heat. The heat is dis- 
sipated by infrared radiation or transported 
away by strong winds in the upper atmosphere. 

The Chemistry of the Aurora 

The aurora is caused by the interaction of high- 
energy particles (usually electrons) with neu- 
tral atoms in the Earth's upper atmosphere. 
These high-energy particles can "excite" (by 
collisions) valence electrons that are bound to 
the neutral atom. The excited electrons can then 
return to their initial, lower energy state, and in 
the process release photons (light particles). 
This process is similar to the discharge in a neon 
lamp (fig. 4). 

Any particular color of the aurora depends on 
a specific atmospheric gas and its electrical 
state, and on the energy of the particle that hits 
the atmospheric gas. Atomic oxygen is respon- 
sible for the two main colors of green (wave- 
length of 557.7 nm) and red (630.0 nm). 



Variations on the Sun 

The Sun is a highly variable star that changes on 
time scales of hours to hundreds of years. The 
interplanetary magnetic field direction and so- 
lar wind speed and density are driven by the ac- 
tivity on the Sun. They can change drastically 
and influence the geomagnetic activity. As geo- 
magnetic activity increases, the southern edge 
of the aurora borealis usually moves to lower 
latitudes. Similarly, solar mass ejections coin- 



The Cause of the Aurora 

Excited oxygen /p\ 
emits radiation 



Red Light 
(630 nm) 







Excited nitrogen 
emits radiation 



Red Light ' y 
(600-700 nm) X 

I 




Figure 4. — Fast electrons from space travel along 
the magnetic field and strike oxygen atoms or nitro- 
gen molecules in the atmosphere. Energy from an 
atom or molecule excited by fast electrons is re- 
leased as different colors of light. 



cide with larger auroral ovals. If the interplane- 
tary magnetic field is in the opposite direction 
of the Earth's magnetic field, there can be in- 
creased energy flow into the magnetosphere 
and thus, increased energy flow into the polar 
regions of the Earth. This will result in an inten- 
sification of the auroral displays. 

Disturbances in the Earth's magnetosphere are 
called geomagnetic storms. These, in turn, can 
produce sudden changes in the brightness and 
motion of the aurora called auroral substorms. 
The magnetic fluctuations of these storms and 
substorms may cause surges in electric power 
lines and occasional equipment failures in the 
power grid, resulting in wide-spread power out- 
ages. They can also impact the performance of 
satellite-to-ground communications and naviga- 
tion systems. Magnetospheric storms can last 
several hours or even days, and auroral sub- 
storms can occur several times a day. Each sub- 
storm can deliver several hundred terajoules of 
energy — as much as the electrical energy con- 
sumed in the entire United States over 10 hours. 



Measuring the Geomagnetic Field 

The geomagnetic field can be measured with 
instruments called magnetometers. Data from 
many magnetometers allow observers to track 
the current state of the geomagnetic conditions. 
The magnetometer data are often given in the 
form of 3 -hourly indices that give a quantitative 
measure of the level of geomagnetic activity. 
One such index is called the K-index. 

The K-index value ranges from to 9 and is di- 
rectly related to the amount of fluctuation (rela- 
tive to a quiet day) in the geomagnetic field 
over a 3-hour interval. The higher the K-index 
value, the more likely it is that an aurora will oc- 
cur. The K-index is also, necessarily, tied to a 
specific observatory location. For locations 
where there are no observatories, one can only 
estimate what the local K-index would be by 
looking at data from the nearest observatory. A 
global average of auroral activity is converted 
to the Kp index. This index is available on a dai- 
ly basis over the World Wide Web. 



Resources 

There are many sources for beautiful pictures 
of the aurora. A few are suggested here, and a 
longer list can be obtained from Space Environ- 
ment Center. 

Gift Shop, Geophysical Institute 
University of Alaska 
Fairbanks, AK 99775-0800 

Astronomical Society of the Pacific 

390 Ashton Ave. 

San Francisco, CA 94112 

National Geophysical Data Center 

NOAA, E/GC2 Dept. 945 

325 Broadway, CO 80303-3328 

info@ngdc.noaa.gov 

Remember also that many resources exist on 
the World Wide Web. Space Environment Cen- 



Written by Larry Combs and Rodney Viereck, 1996 




ter Web page is a good starting point for some 
of these resources: 

http://www.sec.noaa.gov 

References 

Many beautiful pictures have been published in 
articles and books. Here are two: 

Majestic Lights, Robert H. Eather, American 
Geophysical Union, Washington, D.C., 1980 

Aurora Borealis, S.-I. Akasofu, Alaska Geographic 
Society, vol. 6, no. 2, Anchorage, AK, 1979. 

Do-It- Yourself Resources 

You can take your own photos of the aurora if 
you are in a location that permits good viewing 
of the event. This is an excellent article that will 
help you to do that: 

"How to Photograph the Northern Lights," Floyd 
— :on, Alaska magazine, November, 1973.